1. Field
The disclosed concept pertains generally to electrical filters and, more particularly, to inverter filters having differential mode and common mode filter characteristics. The disclosed concept also pertains to systems including an inverter filter.
2. Background Information
As shown in FIG. 1, a voltage source inverter 2 generates a three-phase alternating current (AC) power output 4 using one of many known pulse width modulated (PWM) control algorithms 5. Power is drawn from a direct current (DC) power source 6 and energy is stored in a capacitor bank 8. The DC power source 6 can be, for example and without limitation, a 6-pulse rectifier buffered by AC reactance (not shown) of a three-phase power source 10 and/or by a reactor (not shown) added on the AC side 12 or DC side 14 of the DC power source 6. Relatively larger systems can employ, for example, 12, 18, 24 or more pulses in conjunction with a multi-pulse reactor (not shown) or transformer (not shown) to remove harmonics drawn from the three-phase power source 10.
The voltage source inverter 2 powers a three-phase load 16 through a three-phase output filter 18. Non-limiting examples of the three-phase load 16 include an induction motor, a permanent magnet motor, a synchronous motor, a transformer structured to change the voltage level of a line-to-line output circuit, and a resistive load bank. Two known non-limiting examples of inverter output filters include a sine filter 40 (shown in FIG. 3) and a dV/dt filter 42 (shown in FIG. 4), which only filter the line-to-line power output 4 of the voltage source inverter 2.
Common mode problems arise in low voltage systems from an inverter (e.g., without limitation, the voltage source inverter 2 of FIG. 1) including transistors with parasitic capacitance coupled to ground G. The example load 16 and its load power cables 19 also have parasitic capacitance to ground G. This forms a common mode or zero sequence loop of current flow that is largely orthogonal in terms of energy or power transfer to the intended differential three-phase power circuit. In medium voltage variable frequency drive systems, for example, some topologies also have a ground electrical connection (e.g., a separate ground electrical conductor is included with the three-phase AC power electrical conductors), in order that the three-phase power output is ground referenced.
Typically, when common mode (line-to-ground) problems arise in a variable frequency drive system, a separate common mode filter (e.g., common mode filter 44 of FIG. 3 including three-phase common mode inductor LCM and capacitors CCM) is added to remedy the problem. However, the size and the cost of this separate common mode filter 44 are approximately equal to the size and the cost of the original line-to-line inverter output filter 18 (FIG. 1), the sine filter 40 (FIG. 3) or the dV/dt filter 42 (FIG. 4).
Referring to FIG. 2, in order to have power flow from a load 20 back to an AC power source 22 or from a generator 24 back to a power grid 26, a passive direct current (DC) power source, such as a passive rectifier (e.g., the DC power source 6 of FIG. 1), is replaced by an active rectifier (e.g., without limitation, a PWM rectifier 28). The corresponding power electronics circuit employed for this purpose is typically called an active front end (AFE) 30. The AFE 30, like a voltage source inverter 2 (FIG. 1), generates a square wave and needs to be filtered by an AFE filter 32 before electrical connection to the AC power source 22 or power grid 26. The AFE filter 32 also has significant common mode currents flowing from the active rectifier, such as the PWM rectifier 28, to the AC power source 22 or power grid 26 and back to ground 34.
FIG. 3 shows the known sine filter 40 and the known common mode filter 44. These two separate components can be used in place of the three-phase output filter 18 of FIG. 1. The separate common mode filter 44 can also be used with the AFE filter 32 of FIG. 2 to reduce common mode currents. The capacitor(s) that comprise Cf can be individual (i.e., one per phase), or enclosed in a three-terminal can (not shown) in a wye configuration (not shown).
FIG. 4 shows the known dV/dt filter 42, which, similar to the sine filter 40 of FIG. 3, can be used upstream of the common mode filter 44 of FIG. 3.
There is room for improvement in inverter filters.
There is further room for improvement in systems including an inverter filter.